Mass spectrometric imaging is a technique for investigating the distribution of a substance having a specific mass by performing a mass spectrometric analysis at each of a plurality of measurement points (micro areas) within a two-dimensional area on a sample, such as a biological tissue section. This technique has been increasingly applied in various areas, such as the drug discovery, biomarker search, and identification of the causes of diseases. Mass spectrometers for carrying out mass spectrometric imaging are generally called “imaging mass spectrometers” (see Non-Patent Literature 1, Patent Literature 1 or other documents). They may also be called “microscopic mass spectrometers” or “mass microscopes”, since an analysis using those devices typically includes the steps of microscopically observing a desired two-dimensional area on a sample, setting a measurement target area based on the microscopic observation image, and performing an imaging mass spectrometric analysis on that area. In the present description, the term “imaging mass spectrometer” is used.
An imaging mass spectrometer normally employs an ionization method in which a sample is placed on a sample stage and irradiated with a laser light, electron beam, stream of gas containing charge droplets, plasma gas, etc., to ionize substances (compounds) contained in the sample. Mass spectrometry employing such an ionization method does not require separating the components by a liquid chromatograph (LC), gas chromatograph (GC) or other devices. However, it is often the case that a large number of compounds are simultaneously detected, particularly when the analysis is performed on a biological sample or the like. In such a case, a peak on a mass spectrum which appears to be a single peak may actually be a plurality of peaks derived from multiple compounds and overlapping each other. If a mass spectrometric imaging graphic is created at a mass-to-charge ratio corresponding to such a peak formed by a plurality of compounds overlapping each other, the compound distribution information cannot be accurately obtained, since the signal intensity at each pixel on the mass spectrometric imaging graphic is the sum of the signal intensities which respectively correspond to those compounds.
The rapid technical advancement in mass spectrometers in recent years has led to a dramatic improvement in their mass-resolving power. If such a high-resolution imaging mass spectrometer is used, it is possible to obtain a mass spectrometric imaging graphic which is unaffected by other compounds having close mass-to-charge ratios. However, the improvement in mass-resolving power has also been accompanied by an increase in size and price of the device as well as an increase in the measurement time. In some cases, those restrictions may obstruct the use of a device with high mass-resolving power. There is also the limitation that even a device with the maximally improved mass-resolving power cannot separate different compounds whose mass-to-charge ratios are exactly the same.
One method for solving such a problem is to create a mass spectrometric imaging graphic based on the result of an MSn analysis with n being equal to or greater than two. The imaging mass spectrometer described in Patent Literature 1, Non-Patent Literature 1 or other documents is equipped with an ion trap capable of capturing ions. Such a device can select a specific ion as the precursor ion from various ions of sample origin within the ion trap, and dissociate the selected precursor ion by collision induced dissociation (CID). Accordingly, in the case where a mass spectrometric imaging graphic for a target compound needs to be acquired, an MS2 analysis in which the mass-to-charge ratio of an ion originating from the target compound is selected as the precursor ion is performed at each measurement point, and a mass spectrometric imaging graphic is created using intensity information at the mass-to-charge ratio of a product ion originating from the target compound. Even if there is another compound from which a precursor ion having the same mass-to-charge ratio is generated, its product ion normally has a different mass-to-charge ratio. Therefore, by using the intensity information of the product ion, it is possible to obtain a mass spectrometric imaging graphic which is unaffected by other compounds.
However, the amount of one product ion obtained in the MSn analysis is smaller than that of the original precursor ion, since the precursor ion is partially removed in the process of selecting the precursor ion, and since multiple kinds of product ions are normally generated from the precursor ion by the ion-dissociating operation. Accordingly, if the amount of compound to be observed is originally small, the signal intensity of the product ion may become extremely low. In such a case, it may be impossible to satisfactorily recognize the distribution of the target compound on the mass spectrometric imaging graphic created using the product ion.